The invention relates to a process for the operation of an annular combustion chamber and to an annular combustion chamber with numerous circularly arranged premix burners, in which a fuel-air mixture is produced before it is ignited and the fuel-air mixture is used as a hot gas stream for driving at least one turbine stage of a gas turbine plant.
Premix combustion has become established in the combustion of liquid or gaseous fuels in a combustion chamber of a gas turbine. The fuel and the combustion air are premixed as uniformly as possible and then conducted into the combustion chamber. In order to be correct from an environmental standpoint, care is taken to obtain a low flame temperature by means of a large excess of air. Nitrogen oxide formation can be kept low in this manner.
In this connection, annular combustion chambers have become established, providing numerous individual premix burners in a circular arrangement around the rotating components of a gas turbine, with their hot gases supplied directly to the following turbine stage via an annularly constituted flow channel.
A related annular combustion chamber with premix burners for a gas turbine is known, for example, from EP-B1-597 138. The premix burners provided at the head end of the annular combustion chamber are known, for example, from EP-A1-387 532. Double cone burners are used in such premix burners. This kind of premix burner consists essentially of two hollow, conical partial members which are nested in the flow direction. The respective mid-axes of the two partial members are mutually offset. The adjacent walls of the two partial members form, in their length extension, tangential slots for the combustion air, which reaches the interior of the burner in this manner. A fuel nozzle for liquid fuel is arranged adjacent the tangential slots. The fuel is injected into the hollow cone at an acute angle. The resulting conical liquid fuel profile is enclosed by the tangentially inflowing combustion air. The concentration of the fuel progressively decreases in the axial direction because of mixing with the combustion air.
The premix burners can likewise be operated with gaseous fuel. For this purpose, gas inflow openings distributed in the longitudinal direction, the so-called premix perforations, are provided in the region of the tangential slots in the walls of the two partial members. In gas operation, the mixture formation with the combustion air thus already begins in the zone of the inlet slots. It will be understood that a mixed operation with two kinds of fuel is possible in this manner. As homogeneous as possible, a fuel concentration occurs at the burner outlet over the annular cross section involved. A defined cup-shaped backflow zone, at the top of which ignition occurs, arises at the burner outlet.
Now it is known from various documents, for example, Combust. Sci. and Tech. 1992, Vol. 87, pages 329-362, that with a perfectly premixed flame, the magnitude of the backflow zone, which is equally as important as the so-called flame stabilization region, has no effect on the nitrogen oxide emissions. On the other hand, however, the carbon oxide emissions, and also emissions of unsaturated hydrocarbons (UHC), and especially the extinction limits of the respective premix burners, are strongly affected by the size of the backflow zone. This means that the larger the backflow zone is constituted, the more the carbon oxide emissions, the emissions of unsaturated hydrocarbons, and also the extinction limits, decrease. The consequence of this is that with a larger backflow zone, a greater load region of the premix burner can be covered without the flame being extinguished.
Besides the size of the backflow zone, which as explained above has a critical effect on the manner of operation of the individual premix burners, the fuel distribution, i.e., the mixing profile of the fuel/air mixture in the flame stabilization region, also plays a large part. In a manner known per se, the mixing profile between fuel and air within the premix burner is determined by the premix perforation pattern, i.e., the spatial arrangement of the apertures, typically distributed along the air inlet slots and through which premix fuel, preferably premix gas, is injected into the interior of the premix burner.
All the premix burners are normally given identical premix perforation patterns in annular combustion chambers for the operation of a gas turbine. It is found, though, that different operating regions of the gas turbine arise due to the different load conditions of the gas turbine plant and are characterized by strong combustion chamber pulsations, poor burnup with regard to carbon oxide values and unsaturated hydrocarbon values, and also poor transverse ignition behavior of the individual premix burners. It is critical to improve these characteristics of conventional premix burners.
The invention provides a process for the operation of an annular combustion chamber and also a related annular combustion chamber, in which a fuel-air mixture is produced before being ignited and the fuel-air mixture is used as a hot gas stream for driving at least one turbine stage of a gas turbine plant, such that the disadvantages mentioned hereinabove are to be avoided. In particular, measures are to be found which decisively counteract the combustion chamber pulsations which arise. Furthermore, on environmental grounds and the increasingly stringent guidelines regarding emission values, burnup is to become more complete, and the CO, UHC and NOx emissions reduced.
According to the invention, a process for the operation of a combustion chamber with numerous circularly arranged premix burners includes at least one premix burner being operated such that the at least one premix burner has a spatial mixing profile within the fuel-air mixture differing from all the other premix burners.
According to the invention, an annular combustion chamber is provided with at least one premix burner having at least one region in the premix gas perforation in which adjacent premix gas holes have a different distance from one another than in the remaining region of the premix gas perforation.
The invention deliberately breaks the symmetry which is constructionally predetermined by the circular arrangement of a plurality of identically constructed premix burners around the rotating components of a gas turbine plant. Since identically constructed premix burners are usually arranged annularly around the rotating components of the gas turbine plant, and because of their identical constitution they respectively form identical mixing profiles within the individual fuel-air mixtures. The identical mixing profiles are a consequence of the identical premix perforation patterns. As a result, pulsating waves are formed, circulating in certain load regions of the annular combustion chamber, and the pulsating waves have to be specifically suppressed.
If, on the contrary, a deliberate asymmetry is imposed on the conventional symmetrical structure, the symmetry produced by the identical structure of all the premix burners is broken, and thus no circulating pulsation vibrations caused by resonances can occur.
Such an asymmetry is forced according to the invention in that at least one, preferably three or more, premix burners have a different premix perforation, the premix perforation pattern of which differs from all the remaining premix burners. By the deliberate use of premix perforation patterns deviating from the otherwise identically distributed premix perforation pattern, different mixing profiles are produced, and in turn lead to different burnup results. This finally leads to a decisive damping or counteracting of pulsations which otherwise circulate in the annular combustion chamber, circularly constituted in resonant form. In particular, the measures according to the invention lead to the following advantages:
1. more stable flame position
2. lower emissions of CO, UHC, NOx 
3. complete burnup
4. greater operating range without flame extinction
5. improved transverse ignition properties between two adjacent premix burners, and
6. smaller pulsations.